In MOSFET (metal oxide silicon field effect transistor) integrated circuit structures, for example, multiple laterally disposed MOSFETs are typically formed on the top surface of a semiconductor substrate. These multiple individual MOSFETs are interconnected in a desired fashion using electrically conductive lines which are disposed over, but still insulated from, the top surface of the underlying substrate. If such an electrically conductive line were to extend over a region having either two N type semiconductor regions separated by a P type semiconductor region or two P type semiconductor regions separated by an N type semiconductor region, then the overlying electrically conductive line would form the gate of an undesired MOSFET.
To prevent such an undesired MOSFET from turning on, the threshold voltage of the undesired MOSFET is made to be sufficiently higher than the threshold voltages of the desired MOSFETs on the substrate. Accordingly, the switching on and off of desired MOSFETs via the electrically conductive lines will not cause any undesired intervening MOSFETs to turn on. This increasing of the threshold voltage of the undesired MOSFETs may be accomplished in at least two ways: 1) by doping the channel region of the undesired MOSFET to increase the threshold voltage at which inversion of the channel region will occur, and 2) by increasing the thickness of the field oxide insulator between the electrically conductive line (the gate of the undesired MOSFET) and the underlying channel region of the undesired MOSFET. The LOCOS (LOCal Oxidation of Silicon) process is a well known process commonly used to carry out the above described principles to increase the threshold voltage of intervening undesired MOSFETs. The LOCOS process results in an isolation structure comprising a thick oxide layer (commonly referred to as the field oxide) and an underlying doped field implant layer (commonly referred to as the channel-stop or the field implant).
According to one type of LOCOS process, a layer of a material which is relatively impervious to oxygen and water vapor is formed on the top surface of a semiconductor substrate. The material commonly used is silicon-nitride, hereafter simply referred to as "nitride". This nitride layer is selectively etched away in the field regions leaving only the active regions covered by nitride. After the exposed field regions are ion implanted with a channel-stop dopant, a thermal oxidation process is performed to grow a relatively thick field oxide in the exposed field regions. Due to the oxygen and water vapor impervious characteristics of the nitride layer, the surface of the substrate in the active area is not oxidized in this step.
Tensile stress, however, develops at the silicon-to-nitride boundary. This results in a lateral force being applied to the underlying substrate which, if adequately great, can cause defects in the silicon substrate by dislocation of silicon atoms from the silicon crystal lattice. Because transistor performance is degraded when transistors are built in silicon containing defects, usable silicon area on the die is lost and the resulting integrated circuit cannot be made as small as it otherwise could be. A thin layer of silicon dioxide is therefore commonly provided between the nitride layer and the underlying substrate to reduce the transmission of stress from the silicon to nitride boundary to the substrate and thereby to prevent lattice defects from forming in the substrate.
Silicon dioxide, however, is relatively pervious to oxygen and water vapor. As a result, oxygen and/or water vapor may diffuse laterally into the oxide layer from the exposed side edges of the oxide layer at the field region/active region boundary. As a result, silicon underneath the outer portions of the nitride layer is oxidized. Because the volume of silicon dioxide is almost twice the volume of the silicon it consumes, the silicon dioxide growing under the lateral extent of the nitride layer also lifts the lateral extent of the nitride layer. The more the nitride layer is lifted, the greater the opening for oxygen and/or water vapor to diffuse laterally through the oxide. The resulting shape of the oxide is commonly referred to as a "bird's beak" which points in toward the active region.
Texts describing uses of the LOCOS process, recognized problems with the LOCOS process, and variations on the LOCOS process include: Silicon Processing For The VLSI Era, Volume 2: Process Integration, by Stanley Wolf, Lattice Press, 1990, pages 12-45; Semiconductor Integrated Circuit Processing Technology, by Walter Runyan and Kenneth Bean, Addison-Wesley Publishing Company, 1990, pages 108-112; and Integrated Circuit Engineering, Design Fabrication, and Applications, by Arthur Glaser and Gerald Subak-Sharpe, Addison-Wesley Publishing Company, 1977.
In some LOCOS processes, the nitride layer and the underlying silicon dioxide layer are used as a mask to protect the active region during the field implant step. This way the boundaries of the field implant region will be self-aligned with the boundaries of the nitride layer. When the field oxide is grown in a later step, the field oxide will similarly be aligned with the underlying field implant region. Because an unprotected silicon surface may be damaged during ion implantation, the silicon dioxide layer between the nitride layer and the underlying substrate typically extends over the field region as well as over the active region. The field implant is then performed through the silicon dioxide layer in the field regions, the nitride layer serving to define the boundaries of the implanted region. The nitride layer can more easily block the relatively large phosphorous N-field ions, but the smaller boron P-field ions can penetrate a significantly greater distance through the nitride layer. As a consequence, extra masking protection needs to be provided for the active region without losing the self-aligning feature of the active region nitride mask.
A technique has heretofore been used to provide this masking protection for the active regions. A thicker nitride layer adequately thick to block the smaller boron ions is provided. This technique, however, has drawbacks. Providing a thicker nitride layer may introduce greater stress in the underlying substrate, may cause the nitride layer to develop cracks, may result in a so-called "white ribbon" or "Kooi nitride" thinning of the edges of the field oxide, and may present etching problems. Because thinning of the field oxide reduces the voltage at which the underlying silicon will invert, the isolation function of the field oxide is degraded.
FIG. 1A (PRIOR ART) is a cross-sectional view of thick nitride layer 1 and an underlying oxide layer 2 of the prior art. It is the thick nitride layer 1 and the underlying thin base oxide layer 2 which together mask implanted ions before field oxidation during the field implant step.
FIG. 1B (PRIOR ART) is a cross-sectional view of a self-aligned boundary 3 between an active region and a field region. Thick nitride layer 1 and thin base oxide 2 are shown disposed over the top surface of substrate 7 in the active region after the field oxidation step has been performed. A field implant region 5 is shown disposed underneath field oxide layer 6 in the field region. A bird's beak 4 of silicon dioxide at the rightmost extent of field oxide 6 is shown wedging underneath the leftmost extent of thick nitride layer 1 between the nitride layer 1 and the substrate 7. The leftmost extent of nitride layer 1 is therefore shown deformed and bent upward by the bird's beak. The resistance of the thick nitride layer 1 to this deformation causes the nitride layer 1 to exert a force on the underlying silicon substrate in the vicinity of the self-aligned boundary 3. As described above, exerting a force on the substrate has deleterious consequences including the formation of dislocation defects in the silicon lattice of substrate 7. The more the nitride layer resists the deformation, the greater the stress and the greater the force. Because a thicker nitride layer 1 resists the deformation more than a thinner nitride layer, increasing the thickness of the nitride layer commonly results in silicon defects in the silicon substrate around the bird's beak 4.
After the field oxide layer 6 has been grown, the nitride layer 1 and the underlying base oxide layer 2 are removed from the active region, thereby leaving the field oxide layer in the field region.